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Very large eruptions (>50 km 3 ) and supereruptions (>450 km 3 ) reveal Earth’s capacity to produce and store enormous quantities (>1000 km 3 ) of crystal-poor, eruptible magma in the shallow crust. We explore the interplay between crustal evolution and volcanism during a volcanic flare-up in the Taupo Volcanic Zone (TVZ, New Zealand) using a combination of quartz-feldspar-melt equilibration pressures and time scales of quartz crystallization. Over the course of the flare-up, crystallization depths became progressively shallower, showing the gradual conditioning of the crust. Yet, quartz crystallization times were invariably very short (<100 years), demonstrating that very large reservoirs of eruptible magma were transient crustal features. We conclude that the dynamic nature of the TVZ crust favored magma eruption over storage. Episodic tapping of eruptible magmas likely prevented a supereruption. Instead, multiple very large bodies of eruptible magma were assembled and erupted in decadal time scales.
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An appraisal of the observed crystallinities of volcanic materials
There is a broad consensus that magma eruptibility—the ability of magma stored in the subsurface to erupt onto Earth's surface—is strongly controlled by viscosity. Related to this, a critical parameter that controls viscosity is crystallinity. However, there is uncertainty in the crystallinities that distinguish eruptible from non-eruptible magmas, and whether highly crystalline magmas (>60 vol.%) could be erupted in some conditions. An underutilized but important source of information for understanding this relationship is the observed crystallinities in erupted volcanic materials, which by definition represent a set of eruptible magmas. Here we present a compilation of reported crystallinities for nearly 1000 volcanic samples of differing composition, tectonic setting, and eruption style, which provides valuable insight into the fundamental mechanisms which drive eruptions. Overall, the 95th percentile crystallinity value of our full dataset is 57 vol.%, and \>99 % of all non-dome samples have crystallinity ≤53 vol.%. This suggests that 50–60 vol.% crystallinity represents a fundamental limit for eruptibility for most volcanic rocks. Some dome samples are clear exceptions to this and are erupted with considerably higher crystallinities. There is also a significant correlation between crystallinity and whole rock SiO2 content as observed previously, but a shallow slope suggests whole rock and melt silica content have less impact on critical crystallinity for erupted magma than previously thought. Melt viscosity (as a function of SiO2, temperature, and H2O content) and crystallinity both play important roles on increasing effective viscosity, where melt viscosity plays a more important role at low crystal fractions, and crystallinity plays a more important role at crystallinities greater than ~40 vol.%.
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- Award ID(s):
- 1952808
- PAR ID:
- 10518815
- Publisher / Repository:
- Volcanica
- Date Published:
- Journal Name:
- Volcanica
- Volume:
- 7
- Issue:
- 1
- ISSN:
- 2610-3540
- Page Range / eLocation ID:
- 105 to 115
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.30909/vol.07.01.105115
